! Compile using ...
!
! -fpp flag (Intel/Sun compilers)
! -x f95-cpp-input flag (gfortran)
! -cpp flag (g95)

program main
  implicit none

#include "my_header_file.h"
#define y 5.0

  REALTYPE :: x  ! REALTYPE is defined in the header file
  x=y
  print*, x

end program main